Basically I want to cosplay a character that's: "male, slim, has ridiculous weapon". The reason behind this is I want to make the weapon. It will be my first time making a prop, but I have done other hobby engineering things like Autonomous Submarines, so machining is not new to me.

Kirito from Sword Art Online has a pretty impressive sword and very cool wings in the Alfheim Online arc, that might be kind of fun. Inuyasha also has a pretty ridiculous sword, though he probably has a bit more muscle mass than you. Black Star from Soul Eater has pretty cool weapons.

Gun-based weapons: Borderlands 1 & 2 have some incredible gun designs to work with (Zer0 also has a pretty awesome sword that's lit along the blade). If you also want to do armor, Mass Effect. Fallout would be another great game to look into.